Paraffinic oils containing C.sub.10+ (ten carbons or higher) compounds find various uses as lubricating oils, heating oils, jet fuels, etc. One requirement of these oils is that they have a low viscosity or pour point at low temperatures. However, these paraffin mixtures usually contain straight chain or slightly branched paraffins which are waxes at lower temperatures which result in the paraffin mixture having a high pour point or viscosity at the lower temperatures. In order to remove these waxes, various processes are used to dewax the paraffin feed. Dewaxing processes include catalytic cracking where the long chain paraffins are cracked to smaller chain paraffins and isomerization where the straight chained paraffins are isomerized to branched paraffins.
Dewaxing by isomerization is disclosed in U.S. Pat. No. 4,419,220. The isomerization catalyst which is used is a zeolite beta having a silica: alumina ratio of at least 30:1 and having a hydrogenation component such as platinum. It is further disclosed that lower temperatures favor isomerization over cracking and therefore, lower temperatures are preferred. The '220 patent also discloses a preliminary hydrotreating step to remove nitrogen and sulfur compounds in order to improve catalyst performance and permit operation at lower temperatures.
As the '220 patent discloses, a bifunctional catalyst is used in order to achieve high isomerization selectivity. The two functions are hydrogenation and isomerization. Hydrogenation (and dehydrogenation) is carried out by the metal function, e.g., platinum, palladium, nickel, etc., while isomerization is carried out by the acid function, e.g., zeolites, Si--Al, etc.
The specific steps in paraffin isomerization are as follows. First, the paraffin is dehydrogenated on the metal function to give an olefin. The olefin reacts with the acid function to give a n-carbenium ion which is isomerized to an iso-carbenium ion. Next the iso-carbenium ion will be converted to an iso-olefin followed by hydrogenation to an iso-paraffin. Additionally, the iso-paraffins can be cracked on the acid sites to give low molecular weight paraffins and low molecular weight carbenium ions which will in turn be hydrogenated to light paraffins. Competition between hydrogenation and cracking explains product distribution. To maintain the high isomerization activity the hydrogenation function of the catalyst should be high enough to convert the intermediate carbenium ions to iso-paraffins and prevent their cracking. In the presence of sulfur hydrogenation activity will be suppressed owing to formation of metal sulfides with low hydrogenation activity and thus cracking will dominate resulting in low isomerization selectivity and the formation of light paraffins. Accordingly, there is a need for an isomerization catalyst/process which can function well in the presence of sulfur.
Applicant has found a solution to this problem which involves injection of a nitrogen containing compound, e.g., ammonia, into the isomerization reactor and simultaneously increasing the temperature by about 20.degree. C. to about 150.degree. C.
There are several references which disclose the use of nitrogen compounds in refining processes. For example, U.S. Pat. No. 5,419,830 discloses the use of ammonia to control the temperature in the reactor and prevent temperature runaway. U.S. Pat. No. 4,158,676 discloses the isomerization of monocyclic methyl-substituted aromatic hydrocarbon compounds in which nitrogen containing compounds are injected. U.S. Pat. No. 5,275,720 describes a two stage hydrocracking process in which ammonia is injected in the first stage to increase catalyst cracking activity.
In contrast to these references, applicant's process combines the injection of a nitrogen containing compound with an increase in the operating temperature. Increasing the temperature is contrary to the teachings in the art which state that lower temperatures favor isomerization. This results in improved selectivity and sulfur tolerance.